This invention relates to a method for producing L-threonine, and to a plasmid and a microorganism employed in the method.
L-threonine is an amino acid which plays an important role in nutrition for human beings and animals as an essential amino acid, and its demand for pharmaceuticals, foods, fodder potentiating agent is abruptly increasing in recent years.
With respect to an industrical production method of L-threonine, because of existence of stereoisomers similarly as other amino acids, production of only the L-isomer is difficult by chemical synthesis, and it is primarily produced by the fermentation method. As the method for producing it by the fermentation method, there may be included a method by use of an amino acid-requiring strain (Japanese Patent Publications Nos. 3319/1971, 34193/1971, 34194/1971, etc.). Methods for producing it by the precursor fermentation method include, a method by use of homoserine as the precursor (Japanese Patent Publications Nos. 2896/1961, 6590/1963, 8715/1968, etc.).
However, such fermentation methods cannot be said to be industrially advantageous methods for such reasons that cumbersome operations such as sterilization of medium, etc. are required, that there is the problem of by-products and also that the production management is extremely difficult.
On the other hand, among enzymatic production methods which are less expensive in fixed cost and easier in production management than the fermentation method, there have been proposed a method by the use of glycine and acetaldehyde as precursors (Japanese Unexamined Patent Publications Nos. 121491/1981, 116681/1983, etc.), but this method involves formation of a by-product, allotype threonine, and therefore are not practical methods.
Otherwise, for the production method according to the enzyme method, it has been reported to form L-threonine with a reaction mixture in which DL-homoserine is permitted to exist by use of various microorganism cells (Amino Acids, vol. 1, p. 71-74 (1959)).
However, according to these known methods, the amount of L-threonine formed is not yet satisfactory.
The present inventors have intensively studied in order to produce L-threonine industrially at low cost with good yield, and consequently accomplished the present invention.